1. Field of the Invention
This invention relates to a process for removing sulfur present in the form of carbon oxysulfide or carbonyl sulfide from liquid hydrocarbons. More particularly, the present invention relates to a process for the removal of carbonyl sulfide from hydrocarbon feedstocks containing propylene and to the conditioning of the absorbent material used in the process.
2. Description of the Prior Art
Industrial applications of liquid hydrocarbons and particularly, liquified olefinic hydrocarbons, have become more increasingly specialized. The technology as presently developed utilizes highly efficient catalysts to convert these liquified hydrocarbon feedstocks into final product such as polymers. However, these highly efficient catalysts are very sensitive to contaminants, particularly sulfur contaminants, found in these hydrocarbon feedstocks.
In addition to the well known sulfur compounds such as hydrogen sulfide and mercaptans, the hydrocarbon feedstocks normally contain as small quantity of carbonyl sulfide (COS). Usually COS is present to the extent of only several hundred parts per million (ppm) by weight. However, even this small amount is normally beyond the allowable limits of a acceptable product. Since carbonyl sulfide is almost always formed when carbon, oxygen and sulfur or their compounds, such a carbon monoxide, carbon disulfide and the like are brought together at high temperature, this compound is most frequently found in the hydrocarbon feedstocks resulting from thermal and/or catalytic cracking operations, although, in some cases, it has been found in virgin petroleum fractions.
To some extent carbonyl sulfide is not as reactive as its companion in hydrocarbons, hydrogen sulfide. According to Kirk-Othmer's Encyclopedia of Chemical Technology, Vol. 13, pages 384-386, 1954 edition, carbonyl sulfide reacts slowly with the aqueous alkalimetal hydroxides and is only slowly hydrolyzed to carbon dioxide and hydrogen sulfide. This relatively unreactive characteristic of carbonyl sulfide makes it extremely difficult to remove from petroleum streams by conventional desulfurization techniques.
The presence of COS, even at very low concentrations, oftentimes renders olefins valueless for many purposes. For example high purity olefins are required for the satisfactory production of many polymeric products, especially those useful as plastics, including polymers of ethylene, propylene, and the like. As a result, these has been a real need to improve techniques for removing COS from hydrocarbons, especially those used for polymer production.
Some of the known methods for removing carbon oxysulfide (COS) from hydrocarbon streams include the following. In British Patent Specification No. 1,1452,339, published Feb. 5, 1969, the inventors teach a process for the removal of COS from gas mixtures in which unsaturated compounds such as propyne and propadiene are present comprising passing said mixtures in liquid phase at atmospheric or superatmospheric pressures over a substance which contains one or more of the oxides of cadmium, zinc nickel or cobalt supported on a carrier. It is stated that this process reduces the COS concentration to less than one (1) ppm.
U.S. Pat. No. 4,290,879 to Woodall et al., teaches the removal of carbonyl sulfide from propane and other similar liquified petroleum gas products by mixing liquid methanol with the untreated liquified gas and subsequently containing the liquid mixture with solid potassium hydroxide. The COS concentration in reduced to less than one (1) ppm by volume.
U.S. Pat. No. 3,315,003 to Khelghatian, teaches that carbonyl sulfide can be effectively removed from normally gaseous hydrocarbons by first liquefying the hydrocarbons then contacting them with soda-lime. The effluent gas must subsequently be dried to remove the moisture therefrom.
U.S. Pat. No. 3,284,531 Shaw et al teaches that COS can be removed by passing a fluid hydrocarbon through a bed of an anhydrous, weakly basic anion exchange resin.
U.S. Pat. No. 3,282,831 to Hamm, discloses a method for removing COS from a hydrocarbon stream by utilizing an anionic exchange resin which is in the hydroxyl cycle and which is not fully hydrated.
U.S. Pat. No. 2,951,034 to Stuart is directed to the separation of sulfur compounds from petroleum using a special adsorbent material. Petroleum hydrocarbons essentially contain no olefins. The impurities in petroleum hydrocarbons are mercaptans or aromatic sulfur compounds, not oxysulfides, such as COS. This process comprises a desorption step (col. 3., lines 53 to col. 4, line 2 and col. 4, lines 36-42) which indicates that the impurities are adsorbed in this process which is a reversible step, instead of being absorbed which is an irreversible step.
In the Harshaw Catalysts Technical Bulletin No. 781, a suggested start-up procedure for reduced nickel catalysts was disclosed. The only process disclosed for absorption of sulfur compounds from organic feedstocks was removal of di-ethyl-sulfide from Shell Chemie Holland Shellsol K, which is a dearomatized white-spirit, free of olefins. The process was carried out at 175.degree. C. This temperature would polymerize olefins. There is no indication that COS would be removed from an olefinic feedstock by the catalysts disclosed in this reference or, if it could be removed, what operating temperature would remove COS efficiently without polymerizing the olefinic feedstock. Furthermore, this reference states the need for elimination of exotherms to extend the life of the catalyst and to increase productivity. No pretreatment or conditioning method is disclosed which results in avoiding excessive increase in temperature during the removal process. This reference discusses elimination of exotherms in the start-up or activation procedure.
The problems in purifying propylene and the like olefins are singularly complicated by the nearly identical boiling points of propylene and COS which makes COS removal by fractionation unsuitable. As a result, the levels of COS impurity in propylene stocks are often times intolerably high.
Still other disadvantages are encountered in the heretofore known procedures for the removal of COS from hydrocarbons, particularly those to be used for olefin polymerization. For example, some of the established methods introduce water or other contaminants into the hydrocarbon stream, all of which must be removed by additional processing in order to place the hydrocarbon in suitable condition for use. Any such additional processing, as well as any requirements to employ elevated temperatures adds materially and undesirably to the cost of the operation.
None of the above methods can reduce the COS content to less that fifty parts per billion (50 ppb) by weight. Accordingly, it can be seen that there is a need for process to reduce the COS concentration in a hydrocarbon stream to 50 ppb by weight or lower.